1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and more particularly to a sidelight-type spread illuminating apparatus including a light guiding plate with a light source provided on a light incident face of the light guiding plate. In the spread illuminating apparatus, planar illumination light is adapted to exit out from a light output portion of the light guiding plate.
2. Description of Related Art
In a lighting device provided with a liquid crystal display panel, a sidelight-type spread illuminating apparatus (backlight) having compact, environmentally-compatible LEDs, the LEDs being arranged along the light incident face of a light guiding plate, has been widely applied. This type of spread illuminating apparatus has particularly been used in the field of compact portable information devices such as mobile phones. It is known that, in such a spread illuminating apparatus, uneven brightness derived from a bright area in front of each LED and a dark area between adjacent LEDs occurs in the vicinity of the light incident face of the light guiding plate. Conventionally, in order to solve such uneven brightness, there has been proposed a spread illuminating apparatus, in which a light incident prism for diffusing light is formed on the light incident face of the light guiding plate (for example, see Japanese Patent Application Laid-Open No. 10-293202).
Since recent information devices have become much thinner, a strong demand to make a spread illuminating apparatus thinner also increases. To accomplish the above demand, for example, the following spread illuminating apparatus has been proposed. That is, an inclined surface is provided near the light incident face of a light guiding plate as that the thickness of the light guiding plate gradually reduces from the light incident face toward a light output face. With this structure, the thickness of the light incident face of the light guiding plate corresponds to the light output face of each LED. On the other hand, the light output face portion of the light guiding plate is allowed to be further thinner (see, for example, JP-A No. 2008-170739).
According to studies that have been conducted by the inventors, etc., it has been known that, in these thin-type spread illuminating apparatuses, considering uneven brightness produced near the light incident face thereof, not only bright and dark patterns are just produced, but these patterns are changed depending on how to observe the light output face of the light guiding plate. Such changes in the bright and dark patterns typically occur in a following condition. When a viewer observes the light output face in a substantially vertical direction relative to the light output face of the light guiding plate, an area in front of each LED appears bright and an area between the adjacent LEDs appears dark. On the other hand, when a viewer observes the light output face in a nearly horizontal direction relative to the light output face of the light guiding plate, the bright patterns and the dark patterns are reversed to each other (hereinafter referred to as the “reversal phenomenon”).
Such a reversal phenomenon significantly impairs visibility of an object to be illuminated (for example, a liquid crystal display device), and thus it is very important to inhibit the reversal phenomenon to improve the quality of a spread illuminating apparatus. The reversal phenomenon is caused due to the difference of brightness angular distributions. Specifically, light to be guided in front of the LEDs (that is, near the light incident face of the light guiding plate) mainly contributes to the brightness in an area which faces the LED. On the other hand, light to be obliquely guided from both adjacent LEDs mainly contributes to the brightness in an area between the adjacent LEDs. According to studies by the inventors, etc., it has been found that a conventional light incident prism as described, for example, in JP-A No. 10-293202 fails to sufficiently inhibit this reversal phenomenon.
Furthermore, through their researches, it has been found that the reversal phenomenon becomes effectively inhibitable by providing a group of prisms, which include a plurality of prisms on the light output face of the light guiding plate, the prisms extending substantially vertically with respect to the light incident face. However, particularly in a case of JP-A No. 2008-170739 where a thin-type spread illuminating apparatus has an inclined face in the vicinity of the light incident face, there is a problem that leaked light occurs with a great amount at a boundary between the inclined face and the light output face due to the group of prisms. It thus becomes difficult to apply the above group of prisms as common devices for inhibiting the reversal phenomenon.